Drug insurance is one of many factors that can influence medication adherence. The objectives of this thesis were to evaluate the impact of the type of drug insurance (private versus public) on adherence to antihypertensive medications and the cost of medications, and to evaluate the impact of drug reimbursement procedures and out-of pocket expenses on adherence to medications prescribed for chronic diseases. To meet these objectives, two retrospective cohorts were constructed from the RAMQ and reMed databases: a matched cohort of antihypertensive users covered by private or public drug insurance and a cohort of patients covered by private drug insurance who filled at least one prescription for a medication prescribed for a chronic disease. The results have shown that the level of adherence to antihypertensive medications was similar between the two types of drug insurance and that the cost of antihypertensive medications was 28,9% higher for those privately insured. It was also observed that drug reimbursement procedures didn’t affect medication adherence, while the level of out-of-pocket expenses affected it. Patients with the highest out-of-pocket expenses were less adherent (difference: –19.0%; 95% confidence interval [CI]: –24.0 to –13.0), while patients with no out-of-pocket expenses were less adherent (difference: -9.0%; 95% CI: –15.0 to –2.0) than those with low out-of-pocket expenses. The results presented in this thesis have shown that drug insurance affects medication adherence through the characteristics of drug insurance plans.